Immunosuppressive drugs reduce the risk of allograft rejection by inhibiting T cell mediated alloimmunity; however, they also increase the risk of viral infection by interfering with viral-specific T cells. It has recently been demonstrated in experimental animals that acute viral infections increase the risk of rejection by inducing viral-specific memory T cells with cross-specific alloreactivity (heterologous alloimmunity). Memory T cells also recently have been shown to resist many immunosuppressive therapies including therapeutic T cell depletion, a particularly relevant therapy now used in over 50% of US transplant recipients. We hypothesize that latent infections with herpesviruses such as CMV and EBV induce and maintain memory T cells with heterologous alloreactivity. These cells are refractory to immunosuppression in general and T cell depletion in particular, and to the extent that they cross-react with donor antigens, they prevent graft acceptance. Thus, an individual's response to an allograft and their sensitivity to immunosuppressive drugs is shaped by their prior viral exposures. To investigate this and its clinical ramifications we will perform mechanistic studies in controlled mouse models, and translational analyses in human renal allograft recipients. In Aim 1 we will establish the role of latent herpesvirus infection in allograft rejection in mice, specifically examining the impact of viral reactivation, innate immune responses to the virus, and adaptive T cell responses with their attendant heterologous alloreactivity. In Aim 2 we will determine the mechanisms by which virally-induced donor-reactive T cells escape selective deletion during tolerance induction and therapeutic depletion during immunosuppression in mice and study immunosuppressive strategies to control heterologous alloreactivity. In Aim 3 we will identify the degree to which viral specific T cells comprise a heterologous alloreactive T cell repertoire in humans, and determine how this varies with viral exposure. In Aim 4 we will determine the mechanisms by which virally induced donor-reactive T cells escape immunosuppression in humans, and develop strategies for tailoring immune therapies based on a patient's heterologous repertoire. These studies will facilitate a more fundamental understanding of the nature of allograft rejection and in doing so provide evidence facilitating objective and individualized immune therapy for organ transplant recipients.